DIGESTIVE SYSTEM 143 
right of the latter, between it and the duodenum. The ascending 
branch of one loop runs side by side with the descending branch of 
the next following one. 
II. Anticelous.—The second and third loops are closed and 
sharply alternating ; the second is left-, the third is right-handed ; 
the second lies dorsally, consequently its ascending branch runs side 
by side with that of the third. 
III. Plagiocwlous.—The second and often more loops are doubled 
or turned over with the apices like a horseshoe, giving the loops, 
which are generally open, an irregular or convoluted appearance. 
IV. Pericelous.—The second loop is left-handed, open, and 
encloses the third which is generally straight and closed. This 
formation is of especial interest, because it leads quite gradually to 
the 
V. Cyclocelous formation by the conversion of the second and 
third loops into one left-handed spiral. Such a conversion of the 
second and third loops into a spiral occurs in the Limicole, Laridex, 
and Columbx. ach of these families possesses some genera in which 
the spiral is still represented by long, oval, concentric turns, and 
even some genera which still exhibit the periccelous type with the 
two loops in question still separate, distinct, and more or less 
straight. 
Not every spiral, however, is formed by the concentration of 
two loops. In many instances a spiral is produced by one loop 
being curled upon itself, its apex then forming the centre of the 
spiral. To the apex is attached the diverticulum cecum vitelli ; 
this shews that this spiral is produced by the primitive fold of the 
embryonic mid-gut. 
Such is the case in all the Passeres, and since there are only 
three folds formed by the whole gut, the spiral represents the 
middle or second fold ; hence this arrangement may be distinguished 
as mesogyrous. ‘The number of turns in such a spiral depends 
directly upon the length of the intestine ; while in the short-gutted 
_ Sylviz the spiral is just indicated, there are in the Sparrow (with 
an intestinal length of 21 em.) 14 direct and 1 retrograde turn, 
and in Pinicola enucleator (which possesses an intestine of 99 
cm. in length) there are many direct turns. 
It is clear that with an original number of only four loops, the 
conversion of the two middle ones into a single spiral will cause 
such birds as certain Limicolx, Laridx, and Columbx likewise to 
assume the mesogyrous feature; but the position of the diverticle 
on the original third loop, and the relations of these birds, e.g. 
Charadrius and Sterna, shew that this mesogyrous formation has 
been brought about in a way different from that of the Passeres. 
Lastly, the distal portion of any loop originally straight may be 
coiled up into a spiral, while the rest of the loop remains straight, 
